346 Green Composites from Natural Resources
fatigue criterion was proposed to predict the coefcients of the Paris power law
(Pugno et al. 2006).
Natural bers are characteristically discontinuous and tend to be highly twisted
to obtain sufcient strength from the relatively short bers, as Summerscales et al.
(2010) found that the strength of bers reduced with increasing ber length. However,
highly twisted, this has been noted to be inappropriate for composite reinforcement
as enhanced mechanical strength and stiffness arises when the bers are aligned
in the direction of the load. The twisted nature of the yarns also poses problems
in impregnating the bers with viscous thermoplastics such as PP and poly-(lactic
acid)(PLA).
Liu and Hughes ...